1. Technical Field
This invention relates generally to optical projection systems configured to reduce perceived speckle, and more particularly to a laser-based system employing optical feedback in an external cavity to cause multiple lasers to operate in a coherence collapsed state, thereby reducing speckle as perceived by a viewer.
2. Background Art
Laser projection devices facilitate the production of brilliant images created with vibrant colors. The image quality associated with laser-based projection systems is unmatched by systems using conventional projection devices. The advent of semiconductor lasers, such as laser diodes, allows these brilliant images to be created at a reasonable cost, while using small amounts of power. Laser diodes are small, compact, and relatively inexpensive. Further, the light from laser diodes is easily modulated to form complex images.
One practical drawback associated with using lasers in projection systems is the image artifact known as “speckle.” Speckle occurs when a coherent light source is projected onto an imperfect projection medium. As the light is highly coherent, when it reflects off a rough surface, components of the light combine with other components to form patches of higher intensity light and lower intensity light. In a detector with a finite aperture, such as a human eye, these varied patches of intensity appear as speckles, as some small portions of the image look brighter than other small portions. Further, this spot-to-spot intensity difference can vary, which makes the speckles appear to move.
Turning now to FIG. 1, illustrated therein is a prior art system 100 in which an observer 101 may perceive speckle. Specifically, a coherent light source 101, such as a semiconductor-type laser, delivers a coherent beam 104 to a modulation device 103. The modulation device 103 modulates the coherent beam 104 into a modulated coherent beam 105 capable of forming an image. This modulated coherent beam 105 is then delivered to a projection medium, such as the projection screen 107 shown in FIG. 1.
As the projection screen 107 is imperfect, i.e., as it includes tiny bumps and crevices, the reflected light 108 has portions that combine and portions that cancel. As a result, the observer 102 views an image 106 that appears to be speckled. The presence of speckle often tends to perceptibly degrade the quality of the image produced using the laser projection system.
Numerous attempts have been made to control speckle. Prior art speckle reduction systems include attempts to introduce angle diversity into the coherent beam, attempts to introduce polarization diversity into the coherent beam, attempts to introduce wavelength diversity into the coherent beam, and so forth. Other devices employ diffusers, image displacing devices, and other complex systems. Some speckle reduction systems, such as those used with microscopes, employ long lengths of optical fiber in an attempt to stretch the projected light beyond a corresponding coherence length prior to delivering it to a user. A drawback associated with each of these systems is that they add substantial cost and complexity to the overall system design. For instance, time-varying diffusers require moving or vibrating parts that adversely affect the overall system size and complexity. Further, such systems tend to increase the power requirements of the overall system, thereby degrading efficiency.
There is thus a need for an improved speckle-reducing system for use with laser-based projection systems such as those employing semiconductor-type lasers.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.